Power Transmission: Belts, Pulleys, Gears, Chains and Couplings
This section covers Power Transmission: Belts, Pulleys, Gears, Chains and Couplings. The exam tests velocity ratio, slip, gear terminology, chain drive features and coupling alignment.
Core concepts
- Belt drives are quiet and absorb shock but can slip and creep. Pulley speed ratio is inverse to diameter: N2/N1 = D1/D2 when slip is neglected. Crowning a pulley helps keep a flat belt centred; an idler adjusts tension.
- Gear drives give a positive ratio. For spur gears, module m = pitch-circle diameter ÷ number of teeth, and circular pitch p = πm. Velocity ratio = driven speed ÷ driver speed = driver teeth ÷ driven teeth.
- Chain drives do not slip and suit long centre distances, but need lubrication and correct slack.
- Couplings connect shafts. Rigid couplings need accurate alignment; flexible couplings tolerate small misalignment and shock.
Worked example
A 100 mm driver pulley runs at 1440 rpm and drives a 250 mm pulley. Neglecting slip, N2 = N1 × D1/D2 = 1440 × 100/250 = 576 rpm. The driven pulley is larger, so it turns slower and has higher torque. If slip were 2%, the output would be about 564 rpm, so do not ignore slip when the question supplies it.
Traps that cost marks
Do not invert the pulley ratio. Do not use gear teeth ratio upside down. Remember that belt slip reduces output speed; chain stretch is not the same as slip. Misaligned rigid couplings damage bearings.